This application claims the priority of Korean Patent Application No. 10-2005-0007241, filed on Jan. 26, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical filter apparatus, which separates or combines incident light according to wavelengths, and, more particularly, to an integrated optical filter apparatus, which can be used for a miniaturized waveguide and can reduce crosstalk between adjacent signals.
2. Description of the Related Art
In general, optical filters are employed in optical signal transceivers using multiple wavelengths, which separate and receive an optical signal or combine and transmit optical signals via an optical waveguide according to wavelengths. In the meantime, to integrate optical devices with electronic devices and manufacture cheap miniaturized optical devices, studies of silicon-based optical waveguides have recently been conducted. Accordingly, optical filters used together with the silicon-based optical waveguides are required to more efficiently separate a multiple-wavelength signal transmitted via the miniaturized optical waveguides.
FIG. 1 is a schematic diagram of a conventional optical filter apparatus. Referring to FIG. 1, a conventional optical filter apparatus 5 separates an optical signal beam L0 transmitted via an optical waveguide 1 into first and second light beams L1 and L2 having first and second wavelengths λ1 and λ2 to be respectively read by a first photo-detector 2 and a second photo-detector 3. That is, the optical filter 5 reflects a component of the first wavelength λ1 from the optical signal beam L0 to produce the first light beam L1, and transmits a component of the second wavelength λ2 from the optical signal beam L0 to produce the second light beam L2, such that the optical signal beam L0 is separated into the first light beam L1 and the second light beam L2. Accordingly, the first and second light beams L1 and L2 transmitted via the optical waveguide 1 and separated by the optical filter 5 are respectively received by the first and second photo-detectors 2 and 3 to detect optical signals.
Here, to separate an optical signal, the optical filter apparatus 5 is formed by alternately stacking material layers 6 and 7 with different refractive indices (e.g., layers made of SiO2 and Si) in a multiple layer structure. Here, the thickness of each of the material layers 6 and 7 is determined to be about one fourth of the first wavelength λ1. Accordingly, the optical filter 5 can reflect the first light beam L1 of the first wavelength λ1 and transmit the second light beam L2 of the second wavelength λ2.
FIG. 2 is a graph illustrating a relationship between reflectance and wavelength of the optical filter 5 constructed as above.
Referring to FIG. 2, the first wavelength λ1 is 1490 nm and the second wavelength λ2 is 1550 nm. In the graph, since the reflectance of the light of the first wavelength λ1 is 0 dB, the light of the first wavelength λ1 is reflected 100% and is not directed to the second photo-detector 3. However, since the reflectance of the light of the second wavelength λ2 is greater than approximately −13 dB, about 5% of the light is not transmitted through an incident surface 5a of the optical filter 5 but is reflected to be directed together with the light of the first wavelength λ1 to the first photo-detector 2. Accordingly, the light beam of the first wavelength λ1 does not affect signal detection by the second photo-detector 3, but the light beam of the second wavelength λ2 appears as crosstalk when the first photo-detector 2 detects a signal from the first light beam L1, thereby failing to maintain crosstalk at less than −30 dB required for high quality signal detection.